This invention relates to a spline mechanism for use in relation with telescoping tools forming part of a drill string.
In drilling tools such as vibration dampers, jars, bumper subs and offshore drilling subs, it is usual to incorporate a spline mechanism to transmit rotary motion through the tool while permitting a telescoping motion of the tool's tubular parts to occur. That portion of the tool's outer tubular part which helps to form the spline mechanism will be referred to hereafter as the spline barrel while that portion of the inner tubular part which helps to form the spline mechanism will be referred to as the spline mandrel.
The prior art spline mechanisms of interest can be divided into two groups. In the first such group, circumferentially spaced and longitudinally extending ribs are provided along the internal and external surfaces of the spline barrel and mandrel respectively. These ribs intermesh when the tool is assembled to provide rotary interlocking of the spline barrel and spline mandrel. The parts are free to telescope relative to one another. In the second group, matching longitudinally extending grooves are formed in the said surfaces to define passages. Rods or ball bearings are inserted in the passages to provide the necessary rotary connection for the tool.
The spline mechanism can either be a sealed lubricated system, or one exposed to and working in the abrasive drilling mud which fills the well bore. The present invention had to do with a spline mechanism having insertable splines, preferably having a sealed, lubricated environment in which the splines operate.
Spline mechanisms in oil tool service are characterized by certain problems when in use.
The most serious problem is that of extreme wear of the splines. More particularly, the constant telescoping movement of the conventional steel-on-steel splines while they are transmitting torque rapidly degrades their surfaces. Even when operating in an oil bath, the splines sustain extreme wear. We believe that the lubricant is swept away from the sliding steel surfaces as they are pressed together due to torque. This leaves the surfaces dry and allows galling and wear to occur. In addition, irregularities in the matchup of the steel faces leads to concentrations of stress, which further accelerate wear. If the spline mechanism is open to the drilling mud, wear is increased by the presence of abrasive solids which can get between the sliding spline surfaces.
Because of the extensive wear, it is necessary to replace a spline mandrel or barrel or both the mandrel or barrel on a periodic basis at considerable expense. In order to maximize the useful life of the spline mandrel and barrel, it is thus necessary to fabricate the spline mechanism parts to close tolerances. This naturally increases the manufacturing cost of the tool. The maintenance cost associated with replacing worn out splines, mandrels, and barrels is bound to be a significant portion of the maintenance cost of the tools. It is further found that excessive wear of the splines (which occurs particularly in the spline system exposed to the well environment) leads to serious operating problems with the tool, whereby the tool can become jammed in an open position thus becoming inoperative. In some instances, the substantial amount of backlash which arises in the spline system after excessive wear has occurred will give rise to backing off of the threaded joints and consequent serious tool damage.
Another problem experienced in prior art spline systems occurs when the splines are exposed to the well fluids. More particularly, the annular area below the splines tends to become packed with shale and solid particles from the drilling operation. As this space packs in with solids, the tool becomes difficult or impossible to close.
Another problem common to many prior art oil tool spline mechanisms has to do with their relative weakness and instability when in the extended position. For example, a typical structural arrangement involves providing the female spline in the upper segment of the spline barrel. When the spline mandrel of this arrangement is withdrawn from the barrel to the extended position, the mandrel is likely to wobble when rotating, thereby stressing its threaded connection with the main tool mandrel. Parting of the tool at this point is a frequent and troublesome occurrence.
Another problem experienced in known oil tool spline mechanisms, which are operating internal of a tool in a lubricated environment, is that the enclosed splines tend to give off metallic particles or chips as wear progresses. These particles become dispersed throughout the tool and cause damage to the seals and close fitting parts.